The blood forming hematopoietic system, like many other tissues and body systems relies on a long-lived and relatively small population of adult stem cells to replace cells that are lost or damaged during the process of aging. Given the host of age-related changes that occur in the hematopoietic system (Chen, Astle et al. 1999; Geiger, True et al. 2001; Edwards, Howe et al. 2002; Campisi 2003; Penninx, Guralnik et al. 2003; Pinto, Filippi et al. 2003; Linton and Dorshkind 2004; Balducci and Ershler 2005; Chen 2005; Kamminga, Os et al. 2005; Yuan, Astle et al. 2005) and the quantitative and qualitative changes now known to occur in mouse Hematopoietic Stem Cells (HSCs) with age (Geiger, True et al. 2001; Pinto, Filippi et al. 2003; Chen 2005; Kamminga, Os et al. 2005), we hypothesize that the regulatory mechanisms controlling HSCs are altered during the aging process in a way that contributes to or even produces aging phenotypes. In order to elucidate the mechanisms of HSC regulation, a forward genetic investigation was initiated using two genetically distinct inbred mouse strains, C57BL/6 (B6) and DBA/2 (D2). These strains were selected on the basis of striking inter-strain differences in both organismal longevity and HSC properties. Measuring HSC number in a panel of Recombinant Inbred strains generated by crossing the B6 and D2 parental strains allowed us to perform linkage analysis for this complex, quantitative trait of HSC number. The results of the linkage analysis pointed to several genomic regions, termed Quantitative Trait Loci (QTL) on chromosomes 1,2,3,5,18, and X linked to HSC number in either young or old animals (Geiger, True et al. 2001). These QTL represent a genetic network regulating HSC number between B6 and D2 mice. The identification of the specific genes and regulatory mechanisms corresponding to each QTL is underway. The experiments described in this proposal are designed to reveal the identity of the underlying gene within the QTL that was identified on Chromosome 5 (Chr5) and uncover the mechanism through which this gene effects HSC number. Two congenic mouse strains, in which genes from the Chr5 QTL region were exchanged from B6 onto a D2 genetic background and D2 onto B6, serve as the models for isolating and functionally defining the QTG. First, the identity of the gene will be revealed by microarray-based gene expression studies in HSCs from congenic and parental strains. Second, the mechanism of action for this gene will be revealed by quantitatively comparing 4 major aspects of HSC regulation: cell cycle status, cell cycling rate, self-renewal capacity, and degree of cell death, in HSCs from both B6, D2, and congenic strains as well as HSCs induced to overexpress the QTG. While Americans are enjoying a substantial increase in life expectancy, none who live into old age escape the seemingly inevitable physical impairments that accompany aging. [unreadable] [unreadable] In order to meet the medical, financial, and social needs of the ever-growing population of older Americans, we must understand the mechanisms that control the natural aging process. Identification of the genetic regulatory mechanisms that dictate hematopoietic stem cell properties will provide invaluable insight into the mechanisms responsible for age- associated changes in stem cells that, we believe, lead to the functional declines of aging. [unreadable] [unreadable] [unreadable]